With reference to FIG. 1, a two spool low bypass ratio engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, a low-pressure turbine 18, and an exhaust duct 19.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the high pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The high pressure compressor 14 compresses the air flow A directed into it.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high and low-pressure turbines 16, 18 before being exhausted through the duct 19 to provide additional propulsive thrust. The high and low-pressure turbines respectively drive the high pressure compressor 14, and the fan 12 by suitable interconnecting shafts.
The exhaust duct 19 is typically circular in cross-section and therefore force concentrations and excessive hoop stress are avoided. The duct does not need additional support from the airframe and so the duct can be sufficiently isolated from the airframe to avoid any problems arising from differences in thermal expansion/contraction between the duct and airframe.
It is known to provide an exhaust duct with a non-circular cross-sectional profile. For example, in stealth applications it is desirable to reduce the radar visibility of the exhaust duct. A non-circular exhaust duct introduces additional hoop stresses which must either be accommodated by support from the airframe or the duct must be thickened or reinforcement in the form of stringers must be provided. The thickening/reinforcement increases manufacturing costs and engine operating costs due to the increased weight of material. Providing support from the airframe is problematic owing to the differences in thermal expansion/contraction between the exhaust duct and the airframe.